Electric discharge lamp cathode



Oct. 7, 1941. ow 2,258,158

ELECTRIC DISCHARGE LAMP CATHODE Filed Nov 25, 1940 INVENTOR.

Patented Oct. 7, 1941 UNITED STATES PATENT, OFFICE m 1...? $33.2. ........f...."f..ifi.ii

Hygrade Sylvania Corporation, corporation of Massachusetts Application November 2:, 1940, Serial No. 368,855

8 Claims. 176-126) This invention relates to cathodes for electric gaseous discharge lamps and more particularly 1 to the structure of such of said cathodes as have a coating thereon.

An object of my invention is to provide a cathode of such a structure that it will provide a desirable surface area to which the oxide coating may be applied.

Another object is to provide a cathode which will protect the oxide coating from the direct line of the discharge across the lamp.

A further object is to provide a cathode of a structure which will aid in the prevention of vaporization and sputtering of the oxide coating.

Another object isto provide a method of coating said cathode.

Other objects, advantages and features will be apparent from the following specification taken in conjunction with the accompanying drawing in I which:

before it has been coated;

Figure 2 is a detail to a somewhat larger scale of a portion of the coiled-coil cathode after coating but before blowing Figure 3 is a detail to a somewhat larger scale of the coiled-coil cathode showing the coating held between the turns of the minor coil after blowing;

Figure 4 is a detail shown partly in section of a segment of the minor coil of the cathode shown in Figure 3, showing the manner in which the coating is held between the turns of the minor coil. This figure is drawn to an arc of infinite radius for the sake of clarity and is on a greatly enlarged scale.

In the manufacture of electric gaseous discharge lamps, with a luminescent coating on -the inner walls thereof, it has proven desirable to have the cathodes, sealed at the ends thereof, coated with an alkaline earth oxide. A major factor in the life of theselamps is the ability of the cathodes as a whole to stand up under the discharge which takes place across the lamp. More particularly, it seems that the oxide coating'is the most .sensitive portion of the cathode assembly. when the coating is clearly exposed to the direct line of the discharge, a sparking reaction is noted. This is quite perceptible when the lamp is first turned on, and is noted to a lesser degree while the lamp is running. It consists of small particles of the cathode coating flying oi the surface of the cathode.

Other factors in the gradual break-down and disintegration of the cathode coating, aside from the sparking condition, is the vaporization and sputtering of the oxide coating. These three reactions, all aflectin'g the cathode coating, not only indicate thegradual break-down and disintegration of the coating and the resulting loss in emciency of the lamp. but also create a highly undesirable discoloration on the walls of the lamp near the cathodes. The condensation thus created forms a dark: ring around the walls of the lam near the cathodes.

. To protect the cathode coating from these and any other phenomenae that might deleteriously aifect the life and efliciency of the cathode due to the exposure of the cathode coating, I have employed a coiled-coil type of cathode with the coating held between the turns of the'minor coil.

This coiled-coll cathode is formed by winding a wire into a minor coil and then winding the minor coil to form a major coil. When coated. this cathode appears very similar to a single coil. The cathode may be coated in the usual manner. 1. e. by dipping, spraying, or some similar means. However, I have found that by applying the coating by means of an eye-dropper, the coating will flow freely through and into the turns of the coiled coil. This will naturally result in both the major and the minor coils of the coiled-coil cathode becoming filled with coating, as shown in Figure 2, The cathode thus coated is immediately passed over an air jet,v out of which air under pressure is flowing. ,The pressure of the air and the speed at which the cathode is passed through this flow of air is adjusted so that the coating is blown from the major coil and the main body of the coating now remaining completely fills the turns of the minor coil as shown in Figure 3. v

Since the quantity of cathode coating material to be used on different types and sizes of cathodes is usually a predetermined amount, I adjust the diameter of the minor coil so that when the inner turns of the minor coil are completely filled, they will contain the desired amount of cathode coating. The viscosity and the density of the coating solution. are also adjusted so that after the coating has been applied and blown. there will be a thin film of coatin'g on the outer surface of the minor coil and the space within the core of the minorcoil is completely filled with the coating.

Thus I have found, for example, that for a 40 watt fluorescent lamp, a cathode coating of .15

min. viscosity and 36 Baum density weighing a total length of about 236 mm. as the material with which I form the coiled-coil cathode. By winding this wire on a primary mandrel of .006 inch diameter and winding the resulting coil on a secondary mandrel of .024 inch, :1. coiled-coil of about 8.5 to 9.0 mm. in length suitable for 40 watt lamps is obtained.

In order to remove the cathode coating from the outer surface of the wire of the minor coil, I have employed a series of cathode-heatingsteps in which a current flows through the cathodes after they have been placed in lamps and are being exhausted. This causes the coating material to shrink from the outer surface of the coil and form a closely packed coating completely filling the turns of the coil as shown in Figure 4. Th exposed surface of the tungsten wire, substantially free of any coating material, will now serve as the focal point of the discharge, thus insuring good emission characteristics, longer life, and greater efliciency of the cathode.

Figure 4 shows how the cathode coating is closely held and tightly packed in the turns of the minor coll. It is this minor coil which carries the cathod coating. The major coil is formed when this minor coil is wound to form a major coil. With the coating thus protected, the vaporization, sputtering and sparking of the oxide coating will no longer occur for the coating has been removed from the direct line of the discharge. Thus the condensation of cathode coating materials on the walls of the lamp, which would take place if a cathode as shownin Figure 2 were used, will no longer take place if a cathode as shown in Figure 3 is used.

Although I have used as an example the specifications which I use for a 40 watt lamp, it must be understood that when a cathode for a lamp of greater or lesser wattage is desired, the size of the cathode, the diameter thereof, and the quantity, viscosity and density of the coating must all be changed proportionately so as to provide for the accomplishment of the objects and advantases obtained above.

sesame What I claim is: V

1. An electric discharge lamp cathode comprising: a coiled-coil tungsten wire filament having a major and a minor coil; and an alkaline earth oxide coating filling the space withinthe minor coil only.

2. An electric discharge lamp cathod comprising: a coiled-coil tungsten wire filament having a major and a minor coil; and an alkaline earth oxide coating filling, the entire quantity of which is held within and fills the turns of the minor coil.

3. An electric discharge lamp cathode comprising: a coiled-coil tungsten wire filament having a major and a minor coil; and an alkaline earth oxide filling, the entire quantity of which is held within and fills the turns of the minor coil, with the wire on the outside surface of the minor coil and the spac within the major coil, being free from said alkaline earth oxide.

v4. The process of coating a coiled filamentary electrode, said process comprising: applying a coating suspension to the electrode, and blowing oil! such coating as is not located within the turns of the coil.

5. The process of coating a coiled coil filamentary electrode having a major and a minor coil, said process comprising: applying a coating suspension to the electrode, and blowing ofi such coating as is not located within the turns of the minor coil.

6. An electric discharge device cathode comprising: a coiled-coil tungsten wire filament having a major and a minor coil, the turns of each coil being spaced from the other turns of the same coil; and an alkaline-earth oxide coating substantially filling the minor coil only.

'7. The device of claim 2, in which the turns of each coil are spaced from the other turns of the same coil.

8. The device of claim 3, in which the turns of each 'coil are spaced from the other turns of the same coil.

ERWIN F. LOWRY. 

